BLUE CALCITE SKARNS AS POTENTIAL GEM DEPOSITS: EXAMPLES FROM THE CENTRAL METASEDIMENTARY BELT, GRENVILLE PROVINCE, CANADA

Blue calcite skarns in the Central Metasedimentary Belt (Grenville Province) in Ontario and Quebec exhibit characteristics that are vital to the occurrence of gem-quality grossular. These skarns are proximal to felsic plutonic rocks and hosted in marble (generally dolomitic) as veins or vein systems. Blue calcite skarns are known to occur in Ontario and Quebec, Canada; New York and California, USA; Kenya; etc. We present observations from five occurrences near Bancroft (Ontario), Wakefield, and Grenville (Quebec). Sky-blue calcite is a major component of these skarns, although pale green and white calcite also occurs. At one locality, green calcite turns blue after extended exposure to sunlight or ultraviolet radiation. Mineral assemblages, which vary slightly from one occurrence to the other, generally consist of grossular, wollastonite, diopside, and pyrrhotite with minor vesuvianite, graphite, plagioclase, zircon, uvite-dravite, and fluorapatite. Crystals as large as 3-10 cm are common. The largest crystals discovered to date consist of wollastonite, ~30 cm, and grossular, ~20 cm. Some skarns contain considerable amounts of iron in the form of pyrrhotite, yet the associated silicate minerals are not especially enriched in this element (e.g., X_Mg = Mg / (Mg+Fe) = 0.85 for a representative diopside). The low concentration of Fe in silicates is reflected in the colour of grossular, which ranges from colourless to pale yellow, orange, orange-brown, and golden-brown. Grossular gemstones up to 1.37 carats (brownish-orange ‘hessonite’) and 2 carats (colourless) have been reported from the occurrences under study. Given the widespread occurrence of these skarns, together with their similar geologic environments, large crystal sizes, and low Fe concentrations in silicates, we suggest that good potential for skarn-hosted gem grossular deposits exists in high-grade, marble-bearing contact aureoles in many regions of the world.